This invention relates to methods and apparatus for recovering minerals and the like from both surface and subsurface deposits and more particularly relates to improved leaching methods and apparatus for recovering such minerals.
It is well known to employ conventional shaft and stripmining techniques to recover ores containing valuable metals and other elements, and it is also well known to employ conventional reduction processes to isolate and recover these elements from the ores obtained by such mining techniques. It is further well known that most strip mining has a damaging environmental effect on the surrounding area. Shaft mining also damages the surrounding area, and has the further disadvantage of being hazardous and expensive. On the other hand, there has heretofore been no other way to recover these metals and elements which are essential to our modern industrial society, and thus mining operations have for the most part been tolerated by society as necessary and unavoidable.
A further problem arises from the fact that the ores sought to be recovered are usually physically intermingled with other unwanted material (gangue) and thus conventional excavation techniques will necessarily produce a substantial amount of this unwanted material along with the ores sought to be recovered. Conventional reduction techniques will separate a substantial proportion of the ores sought to be recovered, of course, but a proportion will usually remain in the "tailings" which inherently accumulate at or about the mine site.
It has long been recognized that, although these valuable materials may constitute only a relatively small proportion of these tailings, the very fact that the tailings tend to be accumulated in enormous quantities over a period of time means that a substantial amount of valuable material has also been accumulated. Furthermore, such tailings often contain other valuable minerals and elements besides, which were by-passed during the extraction and reduction operations.
It is conventional to treat an accumulation of mine tailings with a leaching solution for the purpose of using oxidation-reduction reactions for separating and recovering a particular element of mineral from the unwanted material or gangue which constitutes the major portion of such tailings. In particular, the leaching solution is poured onto the tailings either continuously or intermittently, whereby the solution tends to seep down into and percolate through the heap until it reaches an impermeable level, whereupon it exits the heap carrying one or more valuable elements in solution. It will be apparent, of course, that the leaching solution chosen for this purpose must be such that it dissolves only the mineral of interest, and not the gangue in which the mineral is found.
Conventional leaching techniques have sometimes been so successful from a commercial standpoint that it has been proposed to employ leaching to recover ores from subsurface deposits as well. In other words, the leaching solution would be injected into the formation by means of boreholes, and would thereafter be withdrawn from the formation through the same or other boreholes after dissolving part or all of the material sought to be recovered. Leaching has not yet proved to be a suitable substitute for conventional mining operations, except under unusual circumstances, partly because of problems involved in effectively withdrawing the leaching solution from the formation, but principally because of the inherently slow rate at which chemical reactions occur when a leaching solution is used to dissolve the mineral of interest.
The rate at which the mineral value of interest tends to be dissolved in a suitable leaching solution is relatively quite slow. It often negates the use of leaching techniques for recovering mineral values from both subsurface deposits and mine tailings.
These disadvantages of the prior art are overcome with the present invention, however, and novel methods and apparatus are provided for enhancing the rate at which a selected ore or other mineral is dissolved in a leaching solution.